The present invention is directed to a process for producing a petrochemical from a hydrocarbon and an oxygen-containing gas in the presence of a suitable catalyst, more particularly to an improved process for the manufacture of a nitrile, such as acrylonitrile, together with hydrogen cyanide. The present invention is directed to the process for quenching the hot gaseous product stream and removing ammonia (NH3) in the reaction effluent. This quenching stops unwanted side reactions that result in polymerized acrylonitrile, acrolein aldehyde, and other undesirable high molecular weight hydrocarbons. This quenching removes particulates and some contaminants from the hot gaseous product stream.
Many petrochemical products are produced by the oxidation of an appropriate hydrocarbon in the vapor phase over a suitable catalyst. For example, unsaturated nitrites are produced by the ammoxidation of a saturated or olefinically unsaturated hydrocarbon with oxygen in the presence of ammonia and an appropriate catalyst. Similarly, alkylene oxides are produced by the oxidation of lower alkanes or alkenes with oxygen in the presence of an appropriate catalyst. The invention is particularly aimed toward the production of acrylonitriles. However, its utility in any process utilizing air or oxygen as a reactant will be appreciated by one skilled in the art.
A popular method for producing nitrites is the so-called ammoxidation method in which an alkene, such as propylene or isobutene, or an alkane is catalytically reacted with ammonia and oxygen at a high temperature in a gas phase in the presence of the catalyst. Air is generally used as the source of the oxygen because of its low cost and ready availability. Ammonia is supplied in excess to maximize nitrites.
The synthesis reaction can be carried out in any suitable reactor, such as a fixed, fluidized or transport bed reactor. The reactions generally take place at very high temperatures. The reactor effluent is a hot gas that comprises the petrochemical product, and generally unwanted byproducts, carbon monoxide, carbon dioxide, water, air, unreacted hydrocarbons, and ammonia. The reaction equipment train generally consists of a reactor, a petrochemical recovery unit such as a scrubber, in which the product is recovered from the reactor effluent gases by means of water or other solvent, means of further purifying the product, and means for further treating the scrubbed effluent gases.
A major problem associated with the gas phase production of a petrochemical by the oxidation of hydrocarbons is that since the reaction is carried out at elevated temperatures, the products and un-converted feedstock continue to react after the product effluent stream exits the reactor. The product polymerizes, reacts with other constituents in the gas stream, and forms other undesirable high molecular weight hydrocarbons. These many and varied side reactions consume product and create waste which must be separated and disposed of. This waste includes nitrile polymers, acrylic acid polymers, polymerized acrylonitrile, acrolein aldehyde, and numerous other undesirable high molecular weight hydrocarbons. Some undesirable reactions are favored at high pH. Due to high pH condensate caused by excess ammonia, effluent below the dew point temperature is particularly vulnerable. The reactions are very fast, and it is desirable to stop these reactions as quickly as possible and prevent high pH reactions which give yield losses. Much of these contaminants are removed in the gas quench operation. The key elements for a successful quench are to reduce the temperature of the gaseous effluent and to remove ammonia. The undesired side reactions stop once the gas has been quenched. Quenching in the subsequent discussion refers to both temperature reduction and to ammonia removal.
A second problem associated with the gas phase production of a petrochemical by the oxidation of hydrocarbons is that since the reaction is carried out at elevated temperatures, there is an ever-present danger of a fire or an explosion in the reactor, or in the equipment or pipelines associated with the reactor. Accordingly, efforts are constantly made to maintain conditions in the reactor and associated equipment such that the mixture remains outside of the flammability range, or at least out of the autoignition range.
The flammability and autoignitability of a gaseous hydrocarbon-oxygen mixture is dependent upon the composition, the pressure, and the temperature of the gaseous mixture. At low temperatures, the gaseous mixture may have a relatively small flammability range, but as the temperature of the mixture rises, its flammability range increases. Also, as the water vapor content rises due to quenching, the flammability falls due to dilution. As the temperature rises, a point is eventually reached at which the mixture becomes autoignitable. When this point is reached, the mixture may ignite and explode, which event can result in damage to equipment and serious injury or death to persons in the vicinity.
U.S. Pat. No. 3,885,928 describes a process for recovery of the hydrocarbon product produced by the ammoxidation process. This process utilized a quench column to cool the reactor effluent. The patent described a quench column as a device for contacting the hot gas with a counterflowing aqueous stream, though it also stated that an aqueous spray contacts the hot effluent gases. A problem with a quench column is that the quenching process takes a long time, relative to the side reactions that are occurring. Quench columns generally take between about 250 milliseconds and about 700 milliseconds or more to cool the gas. This relatively slow quench is in part because the power input to generate the vapor-liquid contact is relatively low.
U.S. Pat. No. 3,936,360 describes a process where gas cooling is accomplished by first cooling the gas with a heat exchanger, and then injecting the quench liquid into the flowing gas in the same direction as the flowing gas, followed by passing the gaseous stream containing acrylonitrile or methacrylonitrile resulting from the quench to an absorber where water and the gases are contacted in concurrent flow to remove substantially all the acrylonitrile or methacrylonitrile. The aqueous stream containing substantially all the acrylonitrile or methacrylonitrile is then passed through a series of distillation columns and separators for separation and purification of product acrylonitrile and derivatives thereof, and quench fluid is obtained from the bottoms of the final product distillation process. The quench process begins in the pipeline leading to the quench column (called a gas washer), wherein water is sprayed into the gas. The gas washer used a spray nozzle near the top of the quench column to introduce water droplets which contact the gaseous stream. The patent calls this a jet washing device in which a large amount of water is circulated and sprayed onto the reactor effluent. The patent clearly shows the spray to be concurrent with the direction of gas flow. If the washing water is not cooled, the water partially flashes in the gas washer.
Other patents describe processes wherein the quench water is obtained from subsequent separation and product purification steps. For instance, U.S. Pat. No. 4,166,008 describes a process that obtains quench water from the bottoms of a second product recovery distillation tower. U.S. Pat. No. 3,936,360 describes a process that obtains quench water from the bottoms of a hydrogen cyanide recovery distillation tower. Hydrogen cyanide is often produced with acrylonitriles.
The present invention provides an improvement in a process and apparatus for manufacturing petrochemicals by the vapor phase oxidation of a hydrocarbon with oxygen in the presence of a suitable catalyst. The invention comprises contacting the gaseous product stream with a quench fluid in a reverse jet scrubber, preferably with upflow of liquid and downflow of gas. A key element of the success for this arrangement is is the greater atomization of liquid that is obtained due to the greater power input available for atomization of the quench liquid. The power input is adjusted by raising the velocity of the gas and liquid. The quench fluid is injected counter current to the gas flow. The quench fluid impacts the gas stream which is flowing at sufficient velocity to reverse the direction of flow of quench fluid. This creates a zone of intense mixing where the gas is quenched very quickly, i.e., in 100 milliseconds or less. This quick quench stops undesired side reactions, in part by removing reactants such as ammonia and in part by cooling the gas. The quench fluid also removes impurities, including heavy polymers and catalyst fines. The quench fluid introduction means is located downstream of the reaction zone, and is preferably located near the outlet of the ammoxidation reactor.
In some applications of the present invention, the liquid flow can be downflow and the gas can be upflow, and in others the flows can be in the horizontal plane or even at an angle to the vertical plane. As long as the flows are opposed and the gas velocity is sufficiently high, the gas will capture the liquid and reverse the flow direction of the liquid and in the process of reversal generate the turbulence needed for atomization. However, in the acrylonitrile/hydrogen cyanide application the preferred embodiment is with upflow of liquid and downflow of vapor. This is because the need to minimize the opportunity for regions of stagnant liquid and poor contact, which can result in corrosion, polymer growth, and bypass.
In one embodiment of the invention the gas quench fluid is provided from an external source. In an alternate embodiment, expended quench fluid is condensed and separated from the petrochemical-depleted gas stream and used as the quench stream. In a third embodiment, the quench fluid is obtained from a process stream emanating from a subsequent recovery or purification step. An essential element for the acrylonitrile/hydrogen cyanide application of the invention is that the quench fluid contains acid to facilitate removal of ammonia from the gas stream.
In a preferred embodiment, the quench fluid that is not vaporized is recirculated. In a more preferred embodiment, impurities and contaminants are removed from the quench fluid prior to recycle. In a most preferred embodiment, the quench fluid is not cooled during recirculation, and the cooling process is essentially adiabatic, with the sensible heat of the hot gaseous stream being converted to latent heat in the form of vapor. Increased recycle of liquid increases the intensity of the quenching operation and reduces the time required. However, the increased recycle comes at the expense of additional pump, and hence the amount of recycle is one of the key variables in the invention.